Field of the Invention
The present invention relates to image capturing apparatuses and control methods thereof, and particularly relates to image capturing apparatuses that perform focus adjustment using an image signal obtained by an image sensor that photoelectrically converts an object image formed by an optical imaging system, and to control methods thereof.
Description of the Related Art
Conventionally, when a digital still camera, a video camera, or the like carries out autofocus (AF), a method is employed in which a focus lens position having maximum high-frequency components in a luminance signal obtained from an image sensor is taken as an in-focus position. A scanning method, described hereinafter, is known as one such method. In the scanning method, an evaluation value (called a “focus evaluation value” hereinafter) is found based on a high-frequency component of a luminance signal obtained from an image sensor while moving a focus lens across the entirety of a focus detection range, and is stored. At this time, a region for obtaining the focus evaluation value (called an “AF frame” hereinafter) is set in the vicinity of the center of the screen, in the vicinity of a region where an object has been detected, or the like, the focus lens position where the focus evaluation value is maximum in the set AF frame is obtained, and that focus lens position is set as the focus lens position to be used in the image capturing operation.
Meanwhile, a variety of automatic focus adjustment methods for high-luminance objects have been known for some time. In Japanese Patent No. 3105334, whether or not an object is a high-luminance object is determined based on the area of a low-luminance section or a mid-luminance section in an image signal of a frame to be shot. As a result of the determination, a contrast signal is used for focusing operations in the case where the object is a typical object, whereas the focusing operations are carried out so that the area of a high-luminance signal is reduced in the case where the object is a high-luminance object, making it possible to perform accurate focusing operations even on high-luminance objects.
Furthermore, a variety of automatic focus adjustment methods for cases where the object is a point light source are known. In Japanese Patent Laid-Open No. 2002-196220, an average brightness of the object is detected through AE processing, and in the case where the brightness is lower than the specified value, an AF search operation is carried out at an appropriate exposure level based on a result of the AE processing, after which a process for detecting an in-focus position where a focus evaluation value is at a peak is executed. In the case where the in-focus position could not be detected by the AF search operation, the AF search operation is carried out again having reduced the exposure amount from the appropriate exposure, thus avoiding saturation of an image signal caused by a point light source. In this manner, a more accurate in-focus position is detected based on a result of performing a plurality of AF search operations, after which a lens is moved to the obtained in-focus position. As such, this technique enables accurate focusing operations to be carried out for objects in which high-luminance point light sources are interspersed, such as night scenes or the like.
CMOS sensors used as the image sensors of image capturing apparatuses such as digital cameras have different exposure timings from line to line. Accordingly, with illumination conditions in which the intensity of the illumination light fluctuates at frequencies such as 50 Hz, 60 Hz, and so on, as with a fluorescent lamp, band-shaped brightness variations that move in the vertical direction will appear in the screen; this is known as “line flicker”. Such line flicker affects the appearance of through-the-lens images displayed in an LCD. Meanwhile, in the case where the lines affected by the line flicker differ for each readout timing, or in other words, in the case where the line flicker fluctuates, the output of the focus evaluation value for each line will fluctuate due to the line flicker fluctuation; as a result, the correct peak position cannot be detected, which affects the accuracy of the AF.
In response to this, a method is known in which flicker is suppressed by setting the shutter speed of the image capturing apparatus to 1/50 seconds, 1/100 seconds, or the like in the case of a fluorescent lamp that uses a 50 Hz AC power source, and to 1/60 seconds, 1/120 seconds, or the like in the case of a 60 Hz AC power source. Although doing so improves the appearance of the through-the-lens image, it does not result in an optimal exposure setting for AF processing, and conversely will sometimes inhibit the AF accuracy. This is because exposure settings that reduce the depth of field by prioritizing an open aperture and suppress noise through lower gain settings are preferable in order to make it easier to detect the peak position during AF processing.
Accordingly, as a method for ensuring AF accuracy when flicker occurs, in Japanese Patent Laid-Open No. 2008-130531, the image capturing frame rate is set to a timing synchronized to the cycle of the flicker, which prevents fluctuations in the line flicker in order to carry out exposure control suited to AF processing even under light sources that cause flicker.
However, in the aforementioned Japanese Patent No. 3105334, the focusing operations are carried out so that the area of a high-luminance signal is reduced in the case of a high-luminance object/a point light source object. As such, the method is suitable for a case where the primary object is configured only of a point light source, but there are cases where accurate focus adjustment cannot be carried out, such as cases where a point light source object and a normal illuminated object are present together. Recent devices have higher numbers of pixels, making it impossible to ignore even slight shifts in focus and increasing demand for more accurate focus adjustment.
The following can be considered for reasons why accurate focus adjustment cannot be carried out:                the influence of the color of the light source; and        a drop in luminance caused by illuminated areas of a normal object blurring, leading to a drop in the area of high-luminance portions.        
Meanwhile, Japanese Patent Laid-Open No. 2002-196220 discloses performing the AF search having reduced the exposure amount beyond the appropriate exposure amount in the case where the in-focus position cannot be detected, and performing the AF search a plurality of times, namely at the appropriate exposure and at an exposure lower than the appropriate exposure, in the case where the brightness is lower than a specified value. However, there is a problem in that in the case of a high-luminance object, a point light source object, or the like, the influence thereof will result in a position that is not the in-focus position being erroneously determined as an in-focus position, and this problem cannot be solved even if measures are taken for cases where the in-focus position cannot be detected, as with Japanese Patent Laid-Open No. 2002-196220. There is a further problem in that performing the AF search a plurality of times, at the appropriate exposure and at an exposure lower than the appropriate exposure, increases the amount of time required for the AF processing.
Furthermore, although the conventional technique disclosed in Japanese Patent Laid-Open No. 2008-130531 does stabilize the focus evaluation value by preventing fluctuations in line flicker, the focus evaluation value output drops within the AF frame in areas that overlap with lines where the exposure amount has dropped due to the line flicker, and thus the accuracy of the AF cannot be ensured. Further still, the appearance of the through-the-lens image displayed in the LCD worsens due to the occurrence of the line flicker.